Memory for computers has evolved continually, both in form and density, since computers have become a consumer electronic product. The personal computer market started about the time that the 16 kilobit (K) dynamic random access memory (DRAM) was the largest selling memory chip. Computers at that time were sold usually with 32 16K chips for 64 kilobytes (KB) of memory, which was soldered to the computer's motherboard. There was not usually much of a path for adding more memory to the motherboard. There were no provisions for taking out the soldered memory when the next generation chip, the 64K, was developed, as the prevailing thought in the industry was that the average computer user would not require more than 64KB of memory.
As the 64K DRAM became the chip of choice for computer manufacturers, it was common to add several 64K DRAMs. Computers often were populated with 16 DRAMs for 128KB of memory, or 32 DRAMs for 256KB of memory. This memory was also soldered in, and there was no way to add more memory to the motherboard of the computer, as, again, few thought that computer users would need more than 256KB of memory. When it became necessary to have more memory, computer users were forced to purchase memory expansion boards which fit into a slot in the computer. These expansion boards were expensive since, in addition to purchasing the memory, users had to pay for other logic components, connectors, sockets, and a printed circuit board (PCB) so the slot memory would function. This slot memory was not only more expensive than motherboard memory, it was slower because it used a bank switching method of memory access rather than being accessed directly by the central processing unit (CPU), as memory on the motherboard is accessed. In addition, the board memory often used the same port addresses as other add-ons such as modem cards and graphics cards. This resulted in multiple types of boards answering calls to the port by the computer, thereby rendering the two boards incompatible and causing the computer to operate unreliably.
The problems caused by soldering memory to the motherboard lead to the development of more flexible ways to add memory to computers so that the needs of different types of computer users could be met. The SIMM was developed to fulfill this need. SIMMs are not soldered to the computer's motherboard, but are inserted into a slot on the motherboard. A single SIMM can hold several DRAMs and often comes with 256KB of memory. Several slots are usually available on the motherboard so computer users can buy additional memory as they need it. To further enhance the flexibility, some computers have provisions for replacing 256KB SIMMs with one megabyte (MB) SIMMs, even though when the SIMMs first came out, 1MB SIMMs were not available.
In addition to its ease of installation, the SIMM also provides a means for packing a large amount of memory into a relatively small area of the computer, since the RAMs on a SIMM are inserted vertically into the computer. RAM manufacturers also profit from the SIMM style of packaging, since it adds more value to the RAMs than it costs to produce. Due to its flexibility and ease of installation, the SIMM has become the standard for personal computer memory in the industry. This form of packaging memory has gone from a means for packaging DRAMs, to also include read-only memory (ROM) and static random access memory (SRAM).
While the SIMM has many advantages for the computer user, it requires several manufacturing process operations. A typical 1MB SIMM has nine 1-megabit (Meg) DRAMs and nine decoupling capacitors, which serve to make the power to the DRAMs more uniform. With 20 pins per DRAM, and two pins to each of the capacitors, there are 198 solder points required to connect the components to the PCB. There are also approximately 20 die to wire connections (typically aluminum to gold wire bonds) and 20 wire to leadframe connections (typically gold wire to gold- or silver-plated lead frame) required for each 1Meg DRAM to connect the silicon die to the leads of the DRAM package, thereby making a minimum of 558 solder and wire bond connections on a typical 1MB SIMM comprising nine 1Meg DRAMs. Additionally, there are some double bonds required for V.sub.CC and V.sub.SS, as well as connections for grounding the substrate.
With the number of solder and wire bond connections on the typical SIMM, it becomes imperative to have an extremely low connection failure rate. Given the number of connections on the average SIMM, a solder joint/wire bond failure rate of just 0.001 percent means that over 50 percent of SIMMs would fail due to a poor solder joint. Making a reliable connection between two surfaces by soldering varies with many factors. Temperature of the solder, temperature of the surfaces to be joined, residue or oxidation on the surfaces that are to be joined (intermetallic compounds or IMC), contaminated solder, and other factors all affect the success of joining two surfaces by soldering.
Solder joints can be inadvertently broken before the SIMM is shipped, for example due to assembly stresses. While those failures occurring during assembly will most often be caught during a functional test before the product is shipped, they cause expensive rework and scrap. Even if a product passes the most rigorous functional testing, it can fail in the field because of a poor solder connection. Shipping itself stresses the part, and the stress that a customer induces on the SIMM while installing it in the computer can cause a poor but functional solder joint to fail. Sometimes a part will operate at room temperature, but will fail at the elevated temperatures found inside the cabinet of a computer. This failure is sometimes due to a wire bond which makes adequate contact at room temperature, but, at elevated temperatures, lifts up off the contact due to a difference of thermal expansion between the two adjoining surfaces.
The electrical characteristics of a SIMM can also be a concern. The nine 20-pin DRAMs and the nine decoupling capacitors on a typical 1MB SIMM require many traces, the actual number depending on the PCB layout. In any case, the large number of traces on a standard sized PCB requires minimal spacing between the traces. As the output drivers within the DRAMs create intermittent current flow on associated conductive traces, the traces behave as inducters, creating voltage surges which have the potential for creating logic errors. With the addition of radio frequency and electromagnetic interferences occurring within the cabinet of the average personal computer, the myriad of potentially logic-damaging transient voltages is compounded. The relatively long traces that connect the DRAM with the edge connector pins on the PCB also slow the memory access times significantly.
A SIMM has an edge connector on the PCB which has several contact pads that are inserted into a socket in a computer, thereby allowing the transfer of data between the module and the computer. This type of connection is just one of many commonly used on memory and logic modules. Single in-line package (SIP) memory modules are similar to SIMMs, but they have metal leads soldered to the edge connectors on the PCB. The leads are either soldered to the computer's motherboard, or inserted into a socket on the motherboard.